Microcatheters are commonly used by physicians to access discreet regions of the arterial vasculature during interventional procedures. They are typically used to facilitate the placement and exchange of guidewires, and may also be used to subselectively infuse or deliver therapeutic agents. Some examples of microcatheters are Skyway® and Minnie® (Vascular Solutions, Inc.), Rebar®, Nautica™ and Echelon™ (ev3, Inc.), Quick-Cross® Select (Spectranetics, Inc.), Tracker® Excel™ (Boston Scientific, Inc.) and Corsair® (Asahi Intecc) all of which are single lumen catheters, with the lumen sized to have a substantially close fit with a guidewire placed therethrough in order to provide support to the guidewire. These devices may be comprised of a single or multiple-layer polymer shaft, or a polymer shaft with a stainless steel braid. To perform perfusion of diagnostic or therapeutic agents, the guidewire must be removed from the lumen, and this may result in a loss of access to the distal vasculature.